Take steps to keep soybean disease at arm’s length

COLUMBUS – Asian soybean rust didn’t show up in Ohio or Indiana in 2005, but a lot of other soybean diseases did. Growers should take steps to limit future damage by those yield robbers, said Anne Dorrance, Ohio State University Extension plant pathologist. Ready or not. “Conditions favor many soybean pathogens, including seedling and root rots, soybean cyst nematode and Phytophthora root rot,” Dorrance said. “During 2005 we also witnessed numerous foliar diseases, frogeye leaf spot, brown spot and downy mildew. Fortunately no soybean rust was found. Dorrance said there are several things producers can do to minimize effects for these diseases in 2006. Be choosy. Variety selection is essential for disease management. Choose varieties that have the best resistance package for your fields, Dorrance recommends. “For Phytophthora choose a variety with Rps1c, Rps1k or Rps3 gene, but also be sure that the partial resistance level – tolerance – is high.” Dorrance said the Ohio Performance Trials use a scoring system of one to nine, “with three generally our best score and nine is dead.” Not all of the companies use the same scoring system, so be sure to read the fine print when choosing varieties. “In our studies, a resistance gene is only effective for about half the population in any given field, so the partial resistance is important to provide the safety net.” Scout your fields. In combating soybean cyst nematode (SCN), pest populations should be monitored in fields to determine the level of infestation. Varieties then should be selected based on field inspections. “If no or low SCN populations are present, then choose other varieties and do not plant a SCN-resistant line in that field,” Dorrance said. “For low-to-moderate levels, choose a SCN-resistant line.” Sources of resistance are PI88788, Hartwig or CystX, she added. For fields where PI88788 has been used for several years, Dorrance urged farmers to monitor the fields because there are reports of SCN reproducing on these varieties. This is to be expected, she explained, as all current sources of resistance “do have some SCN somewhere in the U.S. that can reproduce on these soybeans.” Seed treatments. Seed treatments provide another means of reducing soybean diseases. “For those fields where replant has been an issue in the past, poor drainage, continuous soybeans or short rotations, a seed treatment will give an economic return more often than not,” Dorrance said. For those fields with a history of Phytophthora, she recommends farmers use the highest labeled rate of Allegiance – 1.5 fluid ounces per hundredweight – or Apron XL – 0.64 fluid ounces per hundredweight – to get the best protection. Management. Proper drainage and crop rotation also are important in slowing disease development, Dorrance said. “All of the root rots, especially Phytophthora, require very moist to flooded soil conditions to cause disease,” she said. Improving soil drainage, from tillage to replacing drain tiles, reduces the time the fields are saturated, and reduces the time these pathogens can infect roots. Crop rotation is still one of the most effective disease management strategies for soybean cyst nematode, Dorrance said. Every year that soybeans are not planted in a field reduces the SCN population by one half.